A defect inspecting apparatus for inspecting a defect on the outer peripheral surface of a pipe has been disclosed in Patent Literature 1. The defect inspecting apparatus disclosed in Patent Literature 1 includes a light source for illuminating the outer peripheral surface of the pipe, an image capture device for grabbing the image of the outer peripheral surface of the pipe by receiving the reflection light reflected by the outer peripheral surface of the pipe, and an inspection device for inspecting a defect on the outer peripheral surface of the pipe by processing the captured image grabbed by the image capture device.
Pipes subjected to such a defect inspection may include a steel pipe for oil well casing, tubing and drilling (hereinafter, referred to as an oil well steel pipe) as shown in FIG. 10. In the end portion of an oil well steel pipe 200, an external thread part 201 and a lip part 202 are provided in that order from the inside in the pipe axis direction. The lip part 202 is tapered such that the dimension in the direction perpendicular to the pipe axis direction decreases outwardly (to the pipe end side) in the pipe axis direction.
In using the oil well steel pipe 200, two or more pipes are joined by a joint 210. On the inner peripheral surface of the joint 210, an internal thread part 211 capable of threadedly engaging with the external thread part 201 of the oil well steel pipe 200 and a no-thread part 212 are provided. When the external thread part 201 of the oil well steel pipe 200 and the internal thread part 211 of the joint 210 are threadedly engaged with each other, the outer peripheral surface of the lip part 202 comes into close contact with the no-thread part 212 of the joint 210. By the close contact of the outer peripheral surface of the lip part 202 with the no-thread part 212, oil leakage from between the oil well steel pipe 200 and the joint 210 is prevented when two or more oil well steel pipes 200 are joined in use.
If a defect is present on the outer peripheral surface of the lip part 202, which is in close contact with the no-thread part 212, oil leakage may occur from between the oil well steel pipe 200 and the joint 210. Also, in the case where the oil well steel pipes 200 are joined in use, a high stress may develop on a load face 203 of the external thread part 201 and in a thread bottom face inspection zone 206. The load face 203 is an inside face (on the side opposite to the pipe end side) of opposing side faces in the pipe axis direction of a thread ridge part 207. The thread bottom face inspection zone 206 is a zone of a thread bottom face 204 ranging from a boundary part 205 between the load face 203 and the thread bottom face 204 to a portion 208 of the thread bottom face 204 spaced apart inwardly by a predetermined distance from the boundary part 205 in the pipe axis direction. Since a high stress develops on the load face 203 and in the thread bottom face inspection zone 206 as described above, if a defect is present on the load face 203 or in the thread bottom face inspection zone 206, the oil well steel pipe 200 may be damaged.
For this reason, any defect on the outer peripheral surface of the lip part 202, on the load face 203, and in the thread bottom face inspection zone 206 must be inspected with high accuracy.
As shown in FIG. 10, some oil well steel pipes 200 have the load face 203 inclined inwardly in the pipe axis direction relative to the vertical plane perpendicular to the pipe axis direction. As shown in FIG. 11, the direction of the optical axis of an image capture device 222 that the apparatus of Patent Literature 1 has coincides with the direction perpendicular to the pipe axis direction. The image capture device 222 of the apparatus of Patent Literature 1, in which the direction of optical axis coincides with the direction perpendicular to the pipe axis direction, cannot grab the image of the load face 203 inclined inwardly in the pipe axis direction. Therefore, the apparatus of Patent Literature 1 cannot inspect a defect on the load face 203 inclined inwardly in the pipe axis direction.
Also, if the load face 203 is inclined inwardly in the pipe axis direction, in the pipe axis direction, the load face 203 is present at the same position as a portion close to the boundary part 205 of the thread bottom face inspection zone 206. Due to the presence of the load face 203, the reflection light reflected to the direction perpendicular to the pipe axis direction by the portion close to the boundary part 205 of the thread bottom face inspection zone 206 enters the load face 203. Therefore, the image capture device 222 of the apparatus of Patent Literature 1, in which the direction of optical axis coincides with the direction perpendicular to the pipe axis direction as described above, cannot grab the image of the entire zone of the thread bottom face inspection zone 206 (i.e. cannot grab the image of the portion close to the boundary part 205 of the thread bottom face inspection zone 206). Therefore, the apparatus of Patent Literature 1 cannot inspect a defect over the entire zone of the thread bottom face inspection zone 206 if the load face 203 is inclined inwardly in the pipe axis direction.
Furthermore, Patent Literature 1 does not describe the inspection of any defect on the outer peripheral surface of the lip part 202.